The accurate measurement of ambient fluid (air) flow is becoming increasingly more important in the application and control of many processes, as well as in the research laboratory. One of the major applications is in the field of air sampling, in which an accurate knowledge of the sampled air quality determines the exposure level to various contaminants. The most widely accepted, primary standard method of flow measurement for a gaseous fluid is the bubble flow meter. In the basic form of the bubble flow meter, a soap film is generated from a soap solution, and is propelled by the gas flow under measurement from one end of the flow meter to the other. By timing the rise of the soap film between calibrated volume marks, the volume flow is obtained. Since for all practical purposes, the soap film is massless, it requires almost no force to accelerate the bubble. Furthermore, a seal is always insured by the presence of the bubble. Accordingly, a soap film flow meter comes closest to meeting the unique requirements of the ideal calibrator. However from the standpoint of convenience the use of a positive displacement flow meter in which a piston is reciprocated within a flowtube may have some practical advantages over the bubble flowmeter.
Nevertheless, the measurement of air flow using a positive displacement reciprocating piston flow meter is susceptible to errors due to the following:
a) Initial piston breakaway friction; PA1 b) Acceleration and deceleration of the piston after breakaway (until equilibrium is reached); PA1 c) Running friction within the flowtube; and PA1 d) Fixed pressure loading determined by the mass of the piston. PA1 (a) a hollow flowtube having two opposite open ends; PA1 (b) a piston disposed in said flowtube for movement between a first position relative to one of said open ends and a second position relative to the opposite open end thereof; PA1 (c) means for connecting said flowtube to an external pump for directing a gaseous fluid through said flow meter at a flow rate to be measured by said flow meter; PA1 (d) valve means for controllably reversing the direction of movement of said piston in said flowtube between said first and second position respectively; and PA1 (e) damping means for damping pulsations in gas flow through said flowtube, said damping means comprising at least one elastic diaphragm responsive to pressure pulsations in said flowtube and a porous member contiguous thereto with the porous member having a multiplicity of voids distributed throughout the porous member and a plurality of open channels.
Each of the above conditions present a load to the air flow system being measured. Moreover the suction pump which is used to draw air through the flowtube may cause air pulsations which in a positive displacement flow meter presents a dynamic load to the system. Various techniques for damping the air pulsations have been suggested heretofore with little success.